Printing traditionally has been defined as a technique for applying under pressure a certain quantity of coloring agent onto a specified surface to form a body of text or an illustration. A more contemporary definition of printing describes printing as any of several techniques for reproducing texts and illustrations, in black or color, on a durable surface and in a desired number of identical copies.
Printing has maintained a quasi-monopoly on the transmission or storage of information. Printing has helped give birth to radio, television, film, microfilm, tape recording, and other rival techniques. Printing is used not merely for making books and newspapers, but also for manufacturing textiles, plates, wallpaper, packaging, billboards, and miniature electronic circuits.
Printing processes require a combination of four elements: (1) a printing press, (2) a printing image carrier, (3) printing ink, and (4) a printing substrate. The act of printing or making the impression takes place in the printing press. The printing image contained in the printing image carrier or printing plate is transferred during this act by means of the printing ink to the paper. In this process, the printing image becomes the printed image. All contemporary industrial printing requires a combination of these four elements.
Gravure printing is an example of intaglio printing. Gravure printing uses a sunken or depressed surface for the image. The image or printing areas are cells or wells etched into a copper cylinder and the non-printing areas are the unetched surface of the cylinder. The image cylinder rotates in a bath of ink. The excess ink is wiped off the surface of the image cylinder by a flexible steel doctor blade. The ink remaining in the thousands of recessed cells of the image cylinder forms the image by direct transfer to the paper as the paper passes between the image cylinder and an impression cylinder.
There are three types of gravure printing: conventional gravure, variable area-variable depth gravure, and direct transfer or variable area gravure. Conventional gravure is used for shorter run, high-quality illustration printing. Variable area-variable depth gravure cylinders made by halftone gravure are used for newspaper supplement, magazine, and catalog printing. Variable area gravure is used mainly for packaging printing.
Gravure presses are manufactured to print sheets (sheetfed gravure) or rolls (rotogravure) of paper. Most gravure is printed from rolls. Sunday newspaper magazine sections or supplements, color inserts for newspapers, large mail order catalogs, wallpaper, plastic laminates, and postage stamps are examples of rotogravure printing.
Gravure printing is considered to be excellent for reproducing pictures and providing metallic finishes, but the high cost of making the image cylinder usually limits the use of gravure to long runs. Accordingly, printing methods which can provide the quality of gravure printing but do so less expensively are very desirable.
The most popular, and a more economical, printing process is lithography. Lithography uses the planographic method. In lithography, the printing area and non-printing area are essentially on the same plane of a thin metal plate and the distinction between the areas is maintained chemically. There are two basic differences between offset lithography and other printing processes: (1) offset lithography is based on the principle that grease and water do not mix, and (2) ink is offset first from the printing plate to a rubber blanket, and then from the rubber blanket to a printing substrate.
When a lithographic printing plate is made, the printing area is made grease-receptive and water-repellent (hydrophobic), and the non-printing area is made water, receptive and ink-repellent (hydrophilic). The printing plate is mounted on a plate cylinder of a press which, as it rotates, comes into contact successively with rollers wet by water and rollers wet by ink. The water wets the non-printing area of the printing plate and prevents ink from wetting this area. The ink then wets the printing area and this wet printing area or image is then transferred from the printing plate to an intermediate rubber blanket cylinder. The wet or inked image is then transferred to a printing substrate as the substrate is passed between the blanket cylinder and an impression cylinder.
Transferring a printing image from a printing plate to a rubber blanket before transferring the image to a printing substrate is called the offset principle. Letter press printing and gravure printing can also employ this offset principle but because most lithography is printed in this way, the term offset has become synonymous with lithography. One major advantage of the offset principle is that the soft rubber surface of the blanket creates a clear impression on a wide variety of paper surfaces and other materials with both rough and smooth textures with a minimum of press make-ready. Offset printing can be recognized by a smooth print, as well as by the lack of any impression, ring of ink, or serrated edge which are characteristic of letterpress printing and gravure printing.
Lithographic offset presses are generally described in terms of the number of colors they can print, whether they are sheet or roll fed, and whether or not they are perfecting.
The number of colors a press can print refers to the number of printing units. Each printing unit will be run with a different color and the number of colors describes how many separate colors can be printed on the sheet with one pass through the press. A split ink fountain on one or more of the units results in more colors being layed down during one pass through the press. The design of multi-color presses has followed two basic approaches. As the lithographic process grew, multi-color presses developed primarily by assembling single color units in tandem with transfer devices between units. Two such units are set up in tandem with a transfer device between them to become a four color press. Three such units become a six color press. The range of colors on most presses used for the general run of lithographic work is from single color to six colors. In the metal decorating field there are presses which "roller-coat" lacquer or base tint on the metal blank before it is passed into the first printing unit.
Whether a press is sheet fed or roll fed designates whether the press moves cut sheets through the feeder or is fed by threading a web of paper from a roll through the press. Web press or roll-fed are the common terms for a press which prints on paper fit directly from a roll. Sheetfed presses which utilize a roll sheeter in place of, or to supplement, a sheet feeder are described as roll-fed through a sheeter.
A perfecting press is a press which prints on both sides of the paper in one pass. Perfecting presses may be sheetfed or roll-fed, single-color or multicolor. Most web-fed presses are perfecting multicolor presses of the blanket-to-blanket design. However, if equipped with the necessary white-roll stands, these presses can be used as single-color perfecting.
Sheetfed lithography is used for printing advertising, books, catalogs, greeting cards, posters, labels, packaging, folding boxes, coupons, trading stamps, and art reproductions. Web offset is used for printing business forms, newspapers, preprinted newspaper inserts, advertising literature, catalogs, long-run books, encyclopedias, and magazines.